1. Field of the Invention
The present invention relates to a nitride semiconductor stacked structure and a semiconductor optical device, and methods for manufacturing the same. More particularly, the invention relates to a nitride semiconductor stacked structure and a semiconductor optical device, both formed by use of Group V materials including ammonia and a hydrazine derivative and a Group III material of an organometallic compound and also to methods for manufacturing the same.
2. Description of the Related Art
Recently, extensive studies and developments of semiconductor lasers capable of light emission ranging from a blue region to a ultraviolet region have been made for high densification of optical disks. For GaN compound semiconductors used as such a blue to violet region diode (laser diode being referred to hereinafter as LD), mentions is made, for example, of GaN, GaPN, GaNAs, InGaN, AlGaN, AlGaInN and the like. GaN semiconductor lasers using nitride semiconductors, such as AlGaInN, have been already put into practice.
For a growth method of a nitride semiconductor, it is usual to use NH3 as a Group V material. In this connection, however, where a p-type semiconductor is grown, the hydrogen atom (H) decomposed from NH3 and a P dopant such as, for example, Mg are combined, with the result that the p-type semiconductor layer after the growth exhibits a high resistance. In order to solve this, for example, thermal treatment is carried out after the crystal growth to activate Mg, thereby ensuring a low resistance.
However, when the thermal treatment is carried out, there may be some possibility that nitrogen (N) desorbed from the surface of the p-type semiconductor layer, thereby degrading the crystal. Nitrogen materials, which do not release hydrogen, e.g. hydrazine materials and amine materials, have been used in some case.
For a method of manufacturing a known nitride compound semiconductor, there is disclosed a method wherein ammonia and a hydrazine are both used as a nitrogen material in such a way that a concentration of the hydrazine ranges from 1×10−3 Vol % to 20 vol % and a ratio of a feed of the hydrazine to the total of a feed of the ammonia and the feed of the hydrazine ranges from 1×10−3 vol % to 10 vol %. It is also disclosed that although a carrier gas used may include gases, such as hydrogen, nitrogen, argon, helium and the like, used singly or in combination, a hydrogen concentration in a preferred carrier gas is at 10 volt or below (see, for example, JP-A-9-251957, Paragraph Numbers [0008] and [0012]).
For a known AlGaInN thin film formation method, there is disclosed a method wherein a substrate temperature is raised to 1000° C. and ammonia is introduced in addition to a hydrazine as a Group V material. After one minute, trimethylgallium is introduced as a Group III material to permit a 3 μm thick GaN layer to be formed, followed by stopping the introduction of trimethylgallium and dropping the substrate temperature to 800° C. Thereafter, trimethylgallium, trimethylaluminum and trimethylindium are simultaneously introduced to grow a 0.5 μm thick Al0.45Ga0.5In0.05N layer. Next, the introduction of trimethylgallium, trimethylaluminium and trimethylindium is stopped, after which the substrate temperature is dropped to 300° C. or below, at which the introduction of the hydrazine and ammonia is stopped (see, for example, JP-A-8-56015, Paragraph Number [0031]).
The following method is disclosed as a known method for manufacturing a nitride semiconductor light-emitting device.
To try to improve the high quality of a nitride semiconductor light-emitting device, the growth temperature of a GaN layer is set at a level lower than hitherto known and a difference in growth temperature between the GaN layer and an GaN active layer is demanded to be controlled within 150° C. In an instance of a MOCVD method, any one of a hydrazine, a substitution product thereof and an amine-based nitrogen compound is used and particularly, a nitrogen compound having a high decomposition efficiency at a temperature as low as 700° C. or below is selected therefrom. These may be used in admixture and may contain ammonia. For a Ga source, TMG (trimethylgallium) or TEG (triethylgallium) is used, for an In source, TMI (trimethylindium) is used, and for an Al source, TMA (trimethylaluminium) is used. As an n-type dopant, SiH4 is used and bismethylcyclopentadienylmagnesium is used as a p-type dopant.
Initially, a first buffer layer (low temperature growth layer) of GaN is grown on a c-face sapphire substrate at a low temperature, after which a second buffer layer of GaN, an n-GaN contact layer, an n—AlGaN cladding layer, an n-GaN optical guide layer and a GaInN active layer are grown at a constant growth temperature of 700° C., followed by successively forming, on the GaInN active layer, a p-GaN optical guide layer, an AlGaN barrier layer, a p—AlGaN cladding layer and a p-GaN contact layer in the same manner as in related art at the same growth temperature as used conventionally. (See, for example, JP-A-2004-47867, Paragraph Numbers [0013] and [0023]-[0025] and FIG. 2.)
For a known method of manufacturing a p-type Group III nitride semiconductor, there is disclosed a method wherein in an atmosphere of a mixed gas of monomethylhydrazine and NH3, TMG, TMI, TMA and (EtCp)2Mg are supplied using hydrogen as a carrier gas to permit a 0.6 μm thick p-cladding layer made of a superlattice with a 50 periods structure of alternate 6 nm thick In0.05Al0.24Ga0.71N layer and 6 nm thick In0.2Ga0.80N layer to be grown, followed by raising the temperature to 1050° C. to stack a 0.2 μm thick p-type GaN contact layer (see, for example, JP-A-2002-319743 and Paragraph Number [0085]).
As a known method of manufacturing a p-type Group III nitride semiconductor, there is disclosed a method wherein a c-face sapphire substrate on which an undoped GaN buffer layer has been formed is placed in a reaction furnace of an MOCVD apparatus and N2 gas alone is introduced into the reaction furnace as a carrier gas. Thereafter, the substrate temperature is so raised that when it exceeds 500° C., 2 mmols/minute of trimethylamine used as an N material is introduced and the substrate temperature is kept at 850° C., under which TMG serving as a Ga material is fed to the reaction furnace at a rate of 10 μmols/minute and cyclopentadienylmagnesium (CP2Mg) serving as a p-type dopant is likewise fed at a rate of 25 μmols/minute, followed by growth for 2 hours to form a p-type GaN layer doped with Mg as a p-type impurity.
In addition, there is also disclosed a method wherein a c-face sapphire substrate on which a GaN buffer layer has been formed is placed in a reaction furnace of an MOCVD apparatus and NZ gas alone is introduced into the reaction furnace as a carrier gas. Thereafter, the substrate temperature is so raised that when it exceeds 500° C., 2 mmols/minute of 1,1-dimethylhydrazine used as an N material is introduced and the substrate temperature is kept at 850° C., under which TMG serving as a Ga material is fed to the reaction furnace at a rate of 10 μmols/minute and DMZ serving as a p-type dopant is likewise fed at a rate of 25 μmols/minute, followed by growth for 2 hours to form a p-type GaN layer doped with Zn as a p-type impurity. (See, for example, Japanese Patent Publication No. 3711635, Paragraph Numbers [0027]-[0028] and [0032].)
For a known method of manufacturing a p-type Group IIII nitride semiconductor, there is disclosed a method wherein trimethylgallium (TMGa) and trimethylindium (TMIn) are, respectively, used as starting materials for Ga and In in an MOCVD method and biscyclopentadienylmagnesium (CP2Mg) is used as a p-type dopant, with which a p-GaN layer is grown in an atmosphere of a mixed gas of dimethyl hydrazine (DMHy) and NH3, and U-GaN, N-GaN and MQW are, respectively, formed in an atmosphere of NH3. Although a hydrazine starting material such as DMHy, tertiary butylhydrazine (TBNy) or the like is used for the formation of a GaN or InGaN layer at a lower temperature, the results of experiments are immediately compared with one another in this method, for which p-GaN is grown at a temperature exceeding 1000° C. under similar conditions as in the conventional formation of p-GaN by use of NH3. (See Eun-Hyun Park et al., “As grown p-type GaN growth by dimethylhydrazine nitrogen precursor”, Journal of Crystal Growth 272 (2004)426-431, page 427, Experiments.)
Where ammonia (NH3) is used as a Group V material in a method of growing a nitride semiconductor, the H radical formed from NH3 is taken in the crystal, thereby causing H passivation to occur or lowering an activity rate of a p-type dopant. As a consequence, the resulting p-type semiconductor layer exhibits a high resistance. This needs annealing so as to increase the activity rate of the p-type dopant when NH3 is used, and thus, not only the manufacturing process becomes complicated, but also there is the possibility that nitrogen (N) desorbs from the surface of the p-type semiconductor to degrade the crystal.
On the other hand, when a material incapable of generating a H radical, e.g. dimethylhydrazine (UDMHy), is used as a Group V material in place of NH3 and a Group III material of an organometallic compound is used, carbon (C) is taken in the crystal, with the attendant problem that there is some possibility that the resulting p-type semiconductor layer exhibits a high resistance.